Dietary fibre

Dietary fibre or roughage is the element of food which cannot be digested. It consists of:

• celluloses
• hemicelluloses
• lignin
• pectins

Poor dietary fibre consumption has been linked with the onset of a number of disease states in the 'developed' western world:

• irritable bowel syndrome
• chronic constipation
• diverticular disease
• appendicitis
• haemorrhoids
• hiatus hernia
• gallstones
• colonic carcinoma
• atherosclerosis

The amount of fibre in the diet has a direct influence on the quantity and consistency of stool produced, and the stool transit time. In the Western world, an adult produces between 80 g and 120 g of firm stool each day with a stool transit time of about 3 days. This is in contrast with an adult in the third world, who has a diet that is similar to that of the hunter-gatherer, i.e. whole grains, cereals, legumes and nuts, supplemented by small quantities of meat and fish, who produce between 300 g and 800g of stool per day with a stool transit time of about one and a half days.

Dietary fibre also influences bile salt metabolism: an increasing amount of deoxycholate is formed from cholate.

Guidelines from the British Dietetic Associate (BDA) with respect to the dietary management of constipation in adults note (1):

Fibre supplements: Good Practice statements

• fibre supplement doses above 10 g/d are optimal for increasing the number of people with constipation who have a clinical benefit, improving stool output and reducing the severity of straining (evidence-based recommendation)
• consuming fibre supplements for a minimum duration of 4 weeks is optimal for increasing stool frequency and improving global constipation symptoms of constipation (evidence-based recommendation)
• in people with constipation who experience tolerance issues with fibre, fibre supplement intake may be increased gradually with weekly increments to avoid adverse effects, such as bloating and flatulence (expert opinion recommendation)
• when advising the use of inulin-type fructan supplements in constipation, the possibility of increased flatulence should be discussed (evidence-based recommendation)
• fibre supplements should be accompanied by additional fluid intake where clinically appropriate (expert opinion recommendation)

Reference:

1. Dimidi E, van der Schoot A, Barrett K, Farmer AD, Lomer MC, Scott SM, Whelan K. British Dietetic Association Guidelines for the Dietary Management of Chronic Constipation in Adults. Neurogastroenterol Motil. 2025 Oct 13:e70173.